Electrically conductive coatings are much in demand by the electronics industry. They are used to protect business equipment from electronic interference (EMI/RFI shielding) to forming complex electronic circuitry on printed circuit boards to antennas for radio frequency identification in applications ranging from warehouse storage of merchandise to U.S. Passports. Electrically conductive printings inks, among the various conductive coatings extant, are growing in demand and, in terms of the variety of application methods and compositions needed to meet the application demands, constitute the paradigm of that coatings class.
Thus, electrically conductive inks are used by the printed electronics industry in order to form conductive elements such as RFID antennas or lines on printed circuit boards. Conductive inks in use today consist essentially of an organic polymer matrix containing, dispersed therein, metal powders, metal powder precursors (metallic decomposition compounds), metal coated fine particles or other conductive particles such as graphite powder. The inks may also contain, usually in low concentration, additives whose role is to maintain/improve stability (for instance, setting agents) or control flow to allow easy application (for instance, solvents), etc. The key property of conductivity is adjusted by controlling the concentration of the metallic powders, its value being proportional, albeit not linearly, to the metal/carrier ratio. Conductive ink compositions are usually applied to a substrate using either low speed processes (e.g., screen printing) or high speed processes (e.g., rotary screen, flexography, gravure), the latter offering the advantage of lower processing cost per print.
The majority of conductive ink compositions in use today are solvent-based thick film systems designed for low speed screen printing (See, e.g., U.S. Pat. Nos. 6,322,620, 4,592,961, 5,622,547, 5,653,918, and 6,939,484. One popular method for printing RFID antennas and the like is screen printing silver-based inks on plastic, paper or cardboard substrates, and then heating to drive off solvent and cure or anneal the ink to thereby form conductive lines with a thickness over 10 micrometers. However, such compositions have many drawbacks. In addition to environmental issues associated with solvent-based systems, thick film applications require considerable thermal energy for drying. They frequently require high drying and curing temperatures and relatively long drying and curing times. Moreover, such a process requires the substrate to be highly heat stable to permit removal of the solvent and curing/annealing of the ink. Therefore, paper, cardboard substrates, and low glass transition (Tg) temperature polymeric substrates, are not easily adaptable as substrates because they cannot withstand high temperatures, even though they may be cost-effective. Other conductive ink systems that can be printed using high speed processes (e.g., gravure) also require high temperature curing conditions (typically over 150° C.). See, e.g., US Pat. Pub. No. 2004/0144958 A1.
Aqueous conductive inks and coatings offer significant ecological advantages over solvent-based compositions, as the latter release solvents into the atmosphere on drying. Aqueous conductive inks, however, have hitherto not offered the high conductivity, or low electrical resistivity, achievable with solvent-based formulas. See, e.g., U.S. Pat. Nos. 5,286,415, 5,389,403, 5,492,653, 5,658,499, 5,756,008, 5,855,820, 6,410,637, 6,576,336, and 6,866,799. High conductivity is necessary to insure that the ink will carry a sufficient electric current when cured. While higher conductivity is conventionally achieved by increasing the concentration of the conductive powder (typically silver), this approach often entails performance as well as economic penalties such as decreased shelf-life and increased cost.
Other desired properties of conductive inks include good abrasion and chemical resistance when dried/cured/annealed so that they are not easily scratched or wiped off during subsequent uses. Therefore, the conductive ink should acceptably adhere to the substrate when dried/cured/annealed and resist being wiped off by a solvent. Furthermore, for high-speed printing (e.g. rotary screen, flexography, gravure), it is necessary that the conductive ink have proper rheology and substrate wetting properties to obtain good ink transfer and graphic reproduction. Additionally, the ink should possess good flexibility and thermal stability to withstand the physical deformation to which the substrate may be subjected. A smooth and uniform layer, i.e., having a low surface roughness, is also highly desirable, especially in applications involving antenna construction.
Accordingly, there is a need for an aqueous conductive ink with high conductivity, good printability, low surface roughness, short drying and curing times at low drying and curing temperatures for use with high speed printing processes (e.g., rotary screen, flexography, gravure). Conductive inks with other desired properties would be further advantageous. The present invention provides a family of electrically conductive compositions suitable for use as coatings and especially as high-speed inks, which possess the desirable properties/characteristics.